In U.S. Pat. No. 3,644,700 to Kruppa et al., there is shown a method and apparatus for controlling a square-shaped beam. The beam is employed to both write desired patterns on chips of a semiconductor wafer or mask and to locate each chip relative to a predetermined position through determining the positions of a pair of registration marks for each chip by utilization of the beam. In the aforesaid Kruppa et al. patent, the location of the two registration marks insures that the pattern can be written within the chip.
Because of the accuracy required in applying the beam to a field, the size of each chip site must be limited to that of the writing field so that any beam error therein is within a certain range. Accordingly, the size of the writing fields cannot be enlarged to enable a single pattern to be written within a single writing field, which defines the maximum size of a chip site in the aforesaid Kruppa et al. patent, when the pattern size exceeds the maximum field size within which the beam can be written and still have the beam error within the desired range.
U.S. Pat. No. 3,900,736 to Michail et al., is an improvement of the method and apparatus of the aforesaid Kruppa et al. patent in that it discloses means whereby a single pattern can be written in more than one writing field rather than being limited to one writing field. Thus, the method and apparatus of Michail et al. permit a semiconductor wafer to have continuous patterns larger than the field to which the beam can be applied accurately to be written therein. Michail et al. accomplish this through utilizing a plurality of square or rectangular shaped fields with each field overlying each of the adjacent fields. Thus, each field which is not on the periphery of the wafer, has an overlying relation with four other adjacent fields. In each of the four corners of the field, a registration mark is disposed in the overlying area of the adjacent fields.
While it is desired that each of these registration marks be at a design position so that the registration marks would define a four sided rectangular or square shaped field having the registration marks at their corners, there is usually some slight deviation of each of the marks from its design position since the registration marks are written on the wafer within a certain tolerance. Therefore, the registration marks are normally not at their design positions but at some deviation therefrom. By ascertaining the deviation of each of the four registration marks for a particular field from the design locations for the registration marks, the boundaries of the writing field are located.
Since the beam is being applied in accordance with a predetermined pattern in which the field was deemed to be a perfect square or rectangle, these deviations of the registration marks for the particular field result in the field not being a perfect square or rectangle. Therefore, if the beam were to be applied in accordance with the predetermined pattern, the beam may be applied beyond the boundaries defined by the registration marks and into another field if correction is not made.
While the patterns for writing within a specific field would be such as to insure that the beam is not applied beyond the field even with the deviations of the marks, this cannot be relied upon when writing a single pattern in more than one field. This is because the beam must be applied to each field separately because of the required accuracy of the beam with respect to field size. Thus, each line written by the beam must stop at a specific boundary so that when the beam is applied to the next adjacent field it will be applied as a continuation of the prior location of the beam at the boundary between the two adjacent fields.
Accordingly, to insure that the beam is applied within the boundaries of the field as defined by the actual locations of the registration marks relative to the beam, it is necessary to dynamically correct the position of the beam when it is stepped from one predetermined position to the next within the field so that the beam is applied to an actual position, which is a deviation from the predetermined position, in accordance with the actual site of the field as defined by the four registration marks of the field. By this dynamic correction at each of the predetermined positions, the beam writes the pattern within the field boundaries as defined by the actual locations of the registration marks.
When the fields are written by moving the beam from one field to the next adjacent field in the X direction and to the right, the registration marks in the upper and lower right hand corners of the first field will be the registration marks in the upper and lower left hand corners for the next field. Therefore, these two registration marks define the common boundary between the two fields and function as reference points to which the beam is applied at the next of the adjacent fields. The other boundaries of the field are similarly defined with respect to the registration marks of the other adjacent fields.
It should be understood that reference points could be ascertained relative to the actual locations of the registration marks and used to define the boundaries of the field rather than the registration marks per se. This shift would be accomplished within the computer, but it would not have any effect on the concept of the pattern being written within the field as defined by the four registration marks at the corners of the field.
Through ascertaining the actual location of each of the registration marks of a field, various digital constants can be determined and applied throughout writing of the pattern within the particular field. The digital constants are utilized to correct for translation, magnification, rotation, and distortion of the beam in the X and Y directions. By using the magnetic deflection voltages for each of the X and Y directions at each of the predetermined positions to which the beam is to be applied and then modifying these voltages by the appropriate digital constants for the particular field, correction voltages are applied for both the X and Y directions to a set of electrostatic field plates to shift the beam from the predetermined position to the actual deviated position in accordance with the actual field as defined by the actual location of the registration marks.
As a result of applying the correction voltage to shift the beam, the beam is written within the boundaries of the field since the beam would either be compressed or extended, for example, in each line to compensate for the difference between the predetermined position and the actual position.
The method and apparatus of Michail et al. are particularly useful when it is desired to write a plurality of patterns at different levels of a chip with each level being written at a different time. This enables overlay accuracy between the written fields at various levels on a chip.
This is accomplished through ascertaining the actual location of each of the four registration marks of a field, as previously mentioned, and retaining these actual locations for reference throughout the various levels of pattern writing. If it should be necessary to use a new set of registration marks, these would be written with their actual locations determined with respect to the actual locations of the prior registration marks, which define the field. Thus, by using the digital constants to correct the translation, magnification, rotation, and distortion of the beam in the X and Y directions, the beam can always be shifted from its predetermined position to its actual deviated position irrespective of the level at which the pattern is being written to insure that the pattern at each level has an accurate overlay with the patterns at other levels of the field.
The aforementioned Kruppa et al. and Michail et al. patents describe apparatus which functions successfully to achieve writing in multiple overlay or chip sites where the silicon wafers being written upon are relatively flat. However, silicon wafers tend to warp during hot processing steps utilized in other stages of semiconductor production. Accordingly, the silicon wafers which typically are to be processed through electron beam systems and written upon at multiple chip sites tend to have considerable warp. Said warp causes a given wafer surface to move out of the electron beam focal plane. This, in turn, causes inaccuracies which are not compensated for in either of the Kruppa et al. or Michail et al. systems.
In addition, mask tilt due to handling is another problem which occurs in writing masks with an electron beam system. This problem has not been dealt with in any of the previously described systems.
Accordingly, it is an object of the invention to provide means in context of a system such as that described by Kruppa et al. and Michail et al. to correct the electron beam focus to match a given semiconductor wafer surface at each chip site. Moreover, it is an object of the invention to provide a technique for correcting for mask tilt.